1. Field of the Invention
The present invention relates to a toner bearing member, a developing device including the toner bearing member, an image forming apparatus including the toner bearing member, such as copiers, printers, and facsimile machines, and an image forming method using the toner bearing member.
2. Description of the Background
In an electrophotographic image forming apparatus such as a copier or a printer, an electrostatic latent image formed on an electrostatic latent image bearing member (e.g., a photoreceptor) is developed into a toner image by a toner bearing member, which may or may not contact the electrostatic latent image bearing member directly. The latter development process, often referred to as contactless, includes a number of different individual processes, such as a powder cloud process, a toner projection process, and an electric field curtain process, for example.
In the toner projection process, toner particles on a toner bearing member are forced, or projected, onto the electrostatic latent image bearing member and adhere to an electrostatic latent image formed thereon by applying a voltage stronger than the adhesive force between the toner particles and the toner bearing member.
In the electric field curtain process, the toner bearing member internally contains a plurality of electrodes arranged at regular intervals. By applying an alternating electric field to the electrodes, an alternating non-uniform electric field is generated on a surface of the toner bearing member and forms an electric field curtain. Previously charged toner particles on the toner bearing member are forced by the electric field and projected from the surface thereof. Since the electric field is periodically reversed, the toner particles also periodically change their direction of movement oppositely along the circumferential surface of the toner bearing member. Such a movement of the toner particles on the circumferential surface of the toner bearing member is hereinafter referred to as “hopping”. When the frequency of the alternating electric field is relatively high, the toner particles are likely to consistently suspend near the surface of the toner bearing member, thus forming toner clouds. The toner clouds then adhere to an electrostatic latent image formed on the electrostatic latent image bearing member. While the toner particles are hopping, the adhesive force between the toner particles and the toner bearing member is substantially zero, which means that no force is needed to separate the toner particles from the toner bearing member. In other words, the toner particles can be satisfactorily supplied to the photoreceptor with a low voltage.
Japanese Patent Application Publication No. H03-21967-A discloses a developing device employing an electric field curtain process. The developing device includes a toner bearing member including multiple electrodes, which are covered with an insulating surface protective layer. Owing to the insulating surface protective layer, the charge of toner particles on the toner bearing member is prevented from leaking into the electrodes. Thus, the toner particles are able to satisfactorily hop on the toner bearing member, resulting in formation of toner clouds. However, it requires an extra process for frictionally charging the toner particles in advance because the toner particles are never frictionally charged with the toner bearing member.
By contrast, Japanese Patent Application Publication Nos. 2007-310355-A and 2007-133388-A each disclose a developing device employing an electric field curtain process, in which the surface of the toner bearing member comprises a material capable of frictionally charging toner particles to a proper polarity. Thus, the toner particles can be charged while hopping on the toner bearing member without an extra process for frictionally charging the toner particles in advance.
However, if the toner particles are excessively charged by contact with the surface of the toner bearing member, a strong electrostatic adhesive force may be generated between the toner particles and the surface of the toner bearing member. When the electrostatic adhesive force is stronger than the force causing hopping of the toner particles, the toner particles may stick to the surface of the toner bearing member without hopping, in other words, without forming toner clouds. Consequently, the toner particles may not be charged to a proper polarity, resulting in production of abnormal images.
Moreover, even if toner clouds are formed normally and normal images are produced in the initial stage, the resulting image quality is likely to deteriorate with time. This is because the adhesive force between the toner bearing member and toner particles varies with gradual abrasion of the surface of the toner bearing member, which causes variation in the electric field of the electrodes in the toner bearing member, the surface roughness of the toner bearing member, the toner charge, etc.
Accordingly, a need exists for charging toner particles to the proper polarity while allowing the toner particles to hop on the toner bearing member to satisfactorily form toner clouds for an extended period of time.